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The Body Mass Index is a scale which compares your height to your weight to produce a numeric score.

  • People who score under 18.5 are considered underweight
  • People who score between 18.5 and 24.9 are considered to be at a healthy weight
  • People who score between 25 and 29.9 are considered to be overweight
  • People who score over 30 are considered obese
  • People who score between 35 and 40 are considered highly obese
  • People who score over 40 are considered extremely obese

How to Calculate Your BMI Score

To calculate your BMI score you divide your weight in kilograms by the square of your height in meters.

  • To convert pounds to kilograms you divide by 2.2. For example, a 200 pound person would be 90.91 kg…200/2.2=90.91
  • To convert inches to meters you multiply by 2.5 and then divide by 100. So a 6 foot tall person would be 72 inches tall, or 1.8 meters tall…72x2.5=180…180/100=1.8

A person who was 6 feet tall and 200 pounds would then calculate their BMI score by computing:

90.91/1.8x1.8=28.1

A person who is 6 feet tall and weighs 200 pounds has a BMI score of 28.1 and is considered overweight.

BMI Score and Disease Risk

According to the National Heart Lung and Blood Institute, you can determine your obesity related disease risk by factoring together:

  1. Your BMI score
  2. Your waist circumference
  3. Your personal risk factors for obesity related diseases

Having a waist size greater than 35 inches for women and greater than 40 inches for men increases the risks of obesity related disease, such as type 2 diabetes and cardiovascular disease, as does having one or more personal risk factors, such as:

  • Hypertension
  • High HDL or low LDL cholesterol levels
  • High blood sugar levels
  • High triglyceride levels
  • A family history of premature heart disease
  • Being a smoker1

Limitations of the BMI Score

As a simple tabulation the BMI score does not always effectively indicate obesity related health risks.

  • Because muscle weighs more than fat and because athletic people can accumulate significant healthy lean muscle mass, an active person might have a high BMI score but a very low body fat percentage. In this situation, a high BMI score does not confer the health risks typically associated with obesity.
  • Conversely, because some elderly people will lose a large percentage of lean muscle mass, some older people who score within a normal BMI range may actually have an unhealthy body fat percentage.
  • People of different racial backgrounds may have different obesity related health risks; because of this some nations have set different cut-off standards for the onset of obesity. In Japan, people with a score of 25 or higher are considered obese and in China people with a BMI of 28 or more are considered likewise.2
References
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Page last updated Nov 20, 2014

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